As is known, a toothed belt of elastomeric material comprises a plurality of inextensible longitudinal inserts, a plurality of teeth, alternating with spaces, cantilevered from at least one face of the belt, and a covering of textile material formed by at least one layer of rubberized fabric applied on the spaces and on the teeth.
The two end portions of the fabric are butt-joined by a seam or are mutually superimposed along a small portion of the end portion. The fabric, inter alia, functions to increase the resistance of the tooth which is made of an elastomeric material deformable per se.
In practice, the tooth can be seen as a bracket fixed at one end, having a surface of maximum extension which has transverse stresses acting thereon. According to this analogy, the fixed tooth portion corresponds to the base region nearest the elastomeric material embedding the inextensible inserts oriented longitudinally of the belt, and the load surface corresponds to one of the sides on which act the transverse thrusts transmitted from one of the sides of the pulley groove.
The fabric cooperates with the elastomeric material constituting the tooth and withstands the tooth deformations. Consequently, by appropriately selecting the threads forming the fabric, the driving force of the teeth can be considerably increased with respect to that of a toothing devoid of a covering.
Unfortunately, the seaming area of the fabric represents a weak point in the resistant structure of the tooth. In fact, the seaming area, usually positioned on the sides of the tooth or on its top, is subjected to cyclic rubbing when the tooth engages with and disengages from the pulley groove, with corresponding pressures which may become high, and with a consequent tendency of the end portions of the fabric to undergo undue detachments.
As it can be understood, the enlargement of the seaming area, at first reduced and then progressively increasing, would compromise critically the overall rigidity of the tooth resistant structure and would impair the possibility of transmitting the desired driving force. Further, any detachment between the end portions of the fabric, even if small, would decrease the belt service life, allowing the immediate penetration of substances, e.g. water, able to attack and/or in some way damage the belt inserts.
In general, it is also to be taken into account that in some cases the belt seaming area tends to collapse not only in the above-indicated transitory steps of engagement and disengagement, but also in the longer phases during which one tooth is engaged in the pulley groove for the whole arc traversed during winding of the belt around the pulley.
The reasons why the fabric seaming area in loaded condition would tend to collapse might be explained as follows. Under load, a belt tooth can be schematically represented as a resistant structure defined by a "three-hinge" arc, the first hinge being situated along the central axis of the belt base; the most distant hinge is then connected to those nearest the base by two rods, one of which, being subjected to the transverse load transmitted by the side of the pulley groove, will be placed under compression. When the belt is moved in a sense opposite to the preceding one, the rod, which was subjected to compression, will be instead subjected to tension.
The elastomeric material applied on the profile of the belt tooth, on one of its sides, will behave always as one of the above-indicated rods and therefore will be subjected to a tension state and to a compression state, respectively, depending on whether it is situated at the zone nearest the point of application of the thrust imparted by the side of the pulley groove or at the opposite zone.
It is evident that the seaming area of the fabric applied on either side of the belt is compelled to follow the stresses exerted on the elastomeric material, and since these are tension stresses in either sense of motion of the belt, a continuous tendency of the fabric end portions to move away from each other will exist.
Unfortunately, the demand for higher and higher driving forces increases the forces causing the enlargement of the seaming area of the fabric, with corresponding greater risks of opening of the seam and a consequently reduced service life of the belt.